I've observed many a bad tourist in Paris, usually my fellow Americans :-)  To be a better tourist, read a few tips from reliable sources, eg Rick Steves. Learn a few basics of French - greetings, asking directions, ordering food, saying thank you, handling money. Speak quietly so only the people you are talking to can hear you. Always say "bonjour" when entering a store, and "merci" and "au revoir" when leaving. Don't touch things in French markets and stores...point and ask for help.  If you are in an establishment, ask if it is OK to take a photo. Paris can be so beautiful at every turn, but remember that people are trying to get to work, so don't stop in the middle of the sidewalk to take pictures...move out of the way. And if you don't want to stand out as a tourist, don't wear your bright new athletic shoes. Most of all, be respectful. Forget about the old rumor that the French are rude...they just don't like rude people. Interact with the French with respect for their language and their culture, and you will be a good tourist.

Good tourists know to keep correct change on them, especially when only ordering an expresso.

Good tourists are the ones who never forget that they are guests in the country they are visiting.